In one type of automatic transmission of an automobile that has been developed in recent years, a lock-up clutch is provided for controlling a coupling state between input and output elements of a torque converter, and this lock-up clutch is engaged as needed to directly couple the input and output elements of the torque converter, thereby avoiding an energy loss which would otherwise occur during power transmission through the torque converter.
FIG. 6 is a cross-sectional view showing an example of a torque converter equipped with a lock-up mechanism. The torque converter 190 includes a hydraulic piston 190B within a housing 190A, and a lock-up clutch 190C is interposed between the inner wall of the housing 190A and the hydraulic piston (lock-up piston) 190B.
When the lock-up clutch 190C is being disengaged, an oil is supplied from a control valve (not shown) to both sides of the hydraulic piston 190B, namely, a coupling-side oil chamber 190D and a release-side oil chamber 190E, so that the pressure in the oil chamber 190D is made substantially equal to that in the oil chamber 190E. Upon receipt of a lock-up command, the oil is supplied from an oil pump (not shown) to the coupling-side oil chamber 190D, to thus increase the oil pressure in the coupling-side oil chamber 190D, whereby the hydraulic piston 190B is moved to the left as viewed in FIG. 6, to engage the lock-up clutch 190C.
In FIG. 6, reference numeral 190F denotes a pump impeller, 190G denotes a turbine runner, and 190H denotes a torsion damper for absorbing vibrations during engagement of the lock-up clutch 190C.
When the output torque from the engine is input to the automatic transmission, the lock-up clutch (torque converter clutch), constructed as described above, may be engaged to transmit output from the engine to the automatic transmission while reducing a loss of the engine output. When the output torque from the engine is not input to the automatic transmission, on the other hand, the lock-up clutch need not be engaged, namely, the input and output elements of the torque converter are not directly coupled to each other, so that the torque converter can smoothly transmit the power from the engine to the transmission while using a fluid (oil) contained therein.
In the meantime, the accelerator pedal position (amount of depression of the accelerator pedal) is used for determining whether the output torque from the engine is input to the automatic transmission. Thus, engagement of the lock-up clutch is controlled based on the accelerator pedal position.
More specifically, when the accelerator pedal is depressed (i.e., if the amount of depression of the accelerator pedal is larger than a predetermined low value), it may be determined that the torque is transmitted from the engine toward the automatic transmission. When the accelerator pedal is not depressed (i.e., if the amount of depression of the accelerator pedal is less than the predetermined low value), it may be determined that the torque is not transmitted from the engine toward the automatic transmission.
Accordingly, the torque converter may be controlled so that the lock-up clutch is engaged if the accelerator pedal is depressed, and engagement of lock-up clutch is released if the accelerator pedal is not depressed.
In such an engine, in which the throttle opening completely corresponds to the amount of depression of the accelerator pedal (accelerator pedal position), the above control for engaging the lock-up clutch may be performed based on the throttle opening.
In the case of an engine equipped with an electronic throttle control device (generally called "drive-by-wire"), which electrically controls the throttle valve, however, a target throttle opening is determined based on the accelerator pedal position, and additional information other than the accelerator pedal position, and the throttle opening is controlled based on this target throttle opening. Since the target throttle opening thus determined does not completely correspond to the actual accelerator pedal position, the control for engaging the lock-up clutch cannot be performed based on this throttle opening instead of the accelerator pedal position.
For the reasons as stated above, the engine equipped with the electronic throttle control device performs control for engaging the lock-up clutch based on the accelerator pedal position.
In recent years, motor vehicles equipped with cruise control devices for maintaining a vehicle at a constant-speed have been developed. The cruise control device controls the engine output and shift positions of the automatic transmission so that the vehicle runs at a constant speed which is equal to a set vehicle speed (target vehicle speed), without requiring depression of the accelerator pedal by a driver.
While the vehicle equipped with such a cruise control device is running under control of the cruise control device, namely, while the cruise control device is being in operation, the torque may be transmitted from the engine to the automatic transmission even if the amount of depression of the accelerator pedal is always zero (or less than a predetermined small amount), in other words, the accelerator pedal is kept at its fully released position.
If engagement of the lock-up clutch is controlled based on the accelerator pedal position while the vehicle is running under the cruise control, the lock-up clutch is always placed in the released or disengaged state, and is thus unable to be engaged even in a driving region in which clutch engagement is desired for the sake of reduction of the energy loss. Thus, the use of the lock-up clutch does not lead to energy saving during the cruise control.
Japanese laid-open Patent Publication No. 61-218442 discloses a technique for controlling the lock-up clutch based on the vehicle speed set by the cruise control device and the shift position of the transmission selected by a transmission controller. However, the control of the lock-up clutch disclosed in the above-identified publication does not take account of whether the torque is being transmitted from the engine to the automatic transmission or not, thus making it difficult to achieve a highly efficient control of the lock-up clutch.